Conventionally, a system for compressing and decompressing sounds, frame by frame based on certain fixed-time frame, has been based on the Code Excited Linear Prediction (CELP) method. This method is described in "4 kbps Improved Pitch Prediction CELP Speech Coding with 20 msec Frame" by Masahiro SERIZAWA and Kazunori OZAWA, in "IEICE TRANSACTIONS ON INFORMATION AND SYSTEMS, VOL. E78-D, No. 6, June 1995, P758-763.
This sound compression/decompression method changes-over processing according to the type of sound contained in a specified frame in order to increase the quality of compressed and decompressed sounds; for example, it compresses and decompresses voiced sounds and unvoiced sounds separately. To do so, this method selects and sets some specified bits from and within a bit stream to indicate that special processing will be performed and includes data for the special processing into the remaining bits of the bit stream.
The conventional sound compression/decompression system is described below with reference to the drawings. FIG. 16 is a diagram showing the configuration of the conventional sound compression/decompression system. FIG. 17 is a flowchart showing the flow of sound compression processing on the conventional system. FIG. 18 is a flowchart showing the flow of sound decompression processing on the conventional system. FIG. 19 is a diagram showing the overview of one-frame encoded bit stream data processed on the conventional system.
As shown in FIG. 16, the system uses source (original) sound data 103, encoded data 104 generated by a sound compression processing module, and decompressed sound data 105 generated by decompressing the encoded data. The system has a sound compression processing module 106 which compresses sounds, frame by frame, i.e., one frame at a time. This module comprises source sound data reading means 107, a voiced/unvoiced frame checking means 108, a special processing identification bit checking means 109, a voiced sound frame compression means 110, an unvoiced sound frame compression means 111, and an encoded data output means 112. The system also has a sound decompression processing module 113 which decompresses sounds, one frame at a time. The module comprises an encoded data reading means 114, a special processing identification bit checking means 115, a voiced sound frame decompression means 116, an unvoiced sound frame decompression means 117, and a decompressed sound data output means 118.
In the following description, special processing refers to a case of compression/decompression processing performed by compressing and decompressing voiced sounds and unvoiced sounds separately. However, it should be noted that, in the later description of the present invention, special processing is not limited to this processing. For example, the special processing of the present invention includes unvoiced sound compression efficiency enhancement which is achieved by processing voiced sounds and unvoiced sounds separately.
The conventional sound compression/decompression system shown in FIG. 16 is described with reference to the flowcharts shown in FIGS. 17 and 18.
First, by referring to FIG. 16 and FIG. 17, the following explains how the source sound data 103 is compressed.
The sound compression processing module 106 performs the following steps. When the source sound data reading means 107 reads one-frame source sound data in step S37, the voiced/unvoiced frame checking means 108 checks, in step S38, to determine whether the frame is an unvoiced sound frame or a voiced sound frame. If the source sound data is determined to be a voiced sound frame in step S38, control is passed to step S39 where the special processing identification bit checking means 109 sets bit 120 which represents a voiced sound frame. Then, the voiced sound frame compression means 110 compresses the sound data in step S40 and the encoded data output means 112 outputs the encoded data in step S41. FIG. 19 shows a one-frame bit stream consisting of encoded data.
On the other hand, if the source sound data is determined to be an unvoiced sound frame in step S38, control is passed to step S42 where the special processing identification bit checking means 109 sets bit 119 which represents an unvoiced sound frame. Then, the unvoiced sound frame compression means 111 compresses the sound data in step S43 and the encoded data output means 112 outputs the encoded data in step S41.
Next, by referring to FIG. 16 and FIG. 18, the following explains how the sound decompression processing module 113 in the conventional sound compression/decompression system decompresses the encoded data 104 generated in the above compression processing.
The encoded data reading means 114 reads encoded data in step S44, and the special processing identification bit checking means 115 checks which special processing identification bit, shown in FIG. 19, is set: the bit 119 representing a unvoiced sound frame or the bit 120 representing a voiced sound frame. If the bit 120 representing a voiced sound frame is set, control is passed to step S46. The voiced sound frame decompression means 116 decompresses the sound data and, in step S47, the decompressed sound data output means 118 outputs the decompressed sound data.
On the other hand, if the bit 119 representing a voiced sound frame is set, control is passed to step S48. The unvoiced sound frame decompression means 117 decompresses the sound data and, in step S47, the decompressed sound data output means 118 outputs the decompressed sound data.